The desulfurization of flue gas results in a product containing calcium sulfite and/or calcium sulfate and calcium hydroxide or calcium carbonate. It is known to subject such a desulfurization product to an oxidation and calcining treatment.
Where the desulfurization product is the result of desulfurizing the flue gases resulting from the operation of hard coal fired power plants after the spray drying-absorption process (SDA-process), the product has substantially the following composition:
calcium sulfite 0.5-hydrate: CaSO.sub.3 *0.5 H.sub.2 O PA1 calcium sulfate 0.5-hydrate: CaSO.sub.4 *0.5 H.sub.2 O PA1 calcium hydroxide: Ca (OH).sub.2 PA1 calcium carbonate: CaCO.sub.3, and PA1 traces of hard coal flue ash. PA1 to provide a method for producing a binding agent or a binder mixture which uses as a starting material the products of the desulfurization and other by-products of the flue gases, whereby the binding agent avoids the above described drawbacks or disadvantages; PA1 to provide a product that not only functions as a binding agent, but simultaneously greatly simplifies or even avoids the environmental problems resulting from the desulfurization of flue gases; PA1 to provide a binding material for construction purposes that can be used instead of conventional gypsum based products without requiring any expensive protection measures or needs only protection measures at greatly reduced costs; PA1 to provide a binding material the strength reaction of which is not limited to temperatures below 35.degree. C.; PA1 to provide a binding material in which the strength reaction is even accelerated at higher temperatures; and PA1 to provide a binding agent that has a smaller ability to go into solution in water than that of gypsum.
The safe storage of such a desulfurization product causes substantial difficulties because one must take into account that the product contains not only soluble sulfates, but also sulfitic components which tend to withdraw oxygen from the ground water.
German Patent Publication (DE-PS) No. 3,037,056 (Bloss) discloses the treatment of a product that is obtained by a wet flue gas desulfurization. Such a product contains calcium sulfite 0.5-hydrate and calcium sulfite di-hydrate. The known treatment results in a product that can be used as a cement, whereby an environmentally detrimental storage becomes unnecessary. The preparation of the flue gases resulting from the combustion of fossile fuels involves a treatment in a washer containing a calcium suspension, whereby, among other components a calcium sulfite 0.5-hydrate is produced. Air is added in order to convert the calcium sulfite 0.5-hydrate partially into a calcium sulfate dihydrate containing suspension which is withdrawn from a water circulating circuit and supplied to a concentrator where it is substantially dewatered. The resulting solid still containing a limited amount of remainder moisture is removed while the liquid is preferably returned to the washer.
An article published in "Zement-Kalk-Gips", (Cement-Lime-Gypsum), No. 1/1986, Volume 39, pages 33 to 35 discloses another method for treating a half-dried product resulting from a flue gas desulfurization. The treatment involves an oxidation resulting in a product suitable for building purposes. The product is in the form of an anhydrite (CaSO.sub.4). The known method converts a sulfite containing product by oxidation into a technically usable calcium sulfate which is suitable for use as a sulfate supplier in Portland cement or in blast furnace cement.
It is also known in the plastering art to use binding agents, the strength of which is based on the following reaction: ##STR1## The above mentioned binding agents are distinguished by the fact that in the form of pastes, mortars, or concretes, they lead quickly to a high strength. For example, the above mentioned reaction is employed in the so-called self-leveling flooring plaster based on synthetic calcium sulfate anhydrite (CaSO.sub.4) or based on natural calcium sulfate anhydrite used in connection with underground construction materials, for example in mines or below grade level.
The strength providing phase of the gypsum based product is always the calcium sulfate dihydrate (CaSO.sub.4 .times.2H.sub.2 O) regardless whether the starting material employs calcium sulfate anhydrite (II) (CaSO.sub.4) or calcium sulfate 0.5 hydrate (CaSO.sub.4 *0.5 H.sub.2 O). Calcium sulfate dihydrate (gypsum) has a solubility in water of 1.81 g/l at 3.degree. C. or 2.04 g/l at 50.degree. C. As a result, conventional gypsum based building materials have an increased water sensitivity or moisture sensitivity which limits the use of such products in many applications.
Another disadvantage of the above prior art methods is seen in that the products are dependent in their material strength on the reaction (1) set forth above. As a result, the material strength of these products are subject to narrow limits as far as temperature is concerned, for example, when these products are used as construction materials below ground level or in mines. The temperature limitation applies particularly for the stimulation system of reaction (1) the effectiveness of which diminishes strongly above 35.degree. C.